The present invention relates to a brake system with electronic brake force distribution as it is disclosed, for instance, in German patent application No. 43 37 498. The prior art brake system adjusts the brake force applied to the rear wheel in relation to the brake force applied to the front wheel in order to ensure an ideal brake force distribution. Pressure control valves are arranged in a hydraulic circuit which connects the master cylinder of the brake system with the rear-wheel brakes. There is provision of an auxiliary-power source to furnish supply pressure and a hydraulic booster to adjust the braking pressure and to operate the master cylinder. Interposed between the master cylinder and the control valves is a change-over valve. The change-over valve is adapted to be selectively switched to adopt a first operating position which connects the control valves to the master cylinder and closes the connection between the control valves and the hydraulic booster, and to adopt a second operating position which connects the control valves to the hydraulic booster and closes the connection between the control valves and the master cylinder. When the change-over valve is switched from the first operating position to the second operating position, the control valves control the hydraulic pressure in the rear-wheel cylinders corresponding to the predetermined relation to the front-wheel braking pressure. The commencement of this control depends on whether a defined starting condition is satisfied or not. Defined criteria must be fulfilled: the brake light switch must be activated, the vehicle speed must have exceeded a determined threshold value, and the vehicle acceleration must have fallen below a determined negative threshold value. In addition, the wheel speed of the rear wheel, the braking pressure of which shall be controlled, must remain under the wheel speed of the front wheel on the same vehicle side by a defined amount. If these conditions are satisfied, the electronic brake force distribution will start to operate. Electronic brake force distribution will be disabled when either the brake light switch is deactivated (what means that the driver has removed his/her foot from the brake pedal), or the vehicle acceleration exceeds a negative threshold value of 0.25 g.
Electronic brake force distribution should be performed only when required. Therefore, a deactivation criterion which is independent of the position of the brake light switch is certainly an appropriate supplementation. However, the calculation of a vehicle acceleration at very low wheel speeds involves many errors because the wheel sensors do not produce exact values about the vehicle speed at low speeds. In this case, the deceleration-responsive criterion can be applied with a wide range of divergence only. Further, there are brake systems having starting conditions for an entry into the electronic brake force distribution which are independent of whether or not pedal-operated braking prevails. Thus, electronic brake force distribution will possibly be activated when the vehicle passes a bump or any other disturbance on the road surface which took effect on the individual wheel speed. With such activation criteria, it is necessary to choose deactivation criteria which are not linked to the release of a brake pedal. In rush hours, for example, it is possible that the driver has heavily decelerated the vehicle for a period of time during stop-and-go traffic and thereby triggered electronic brake force distribution. However, because the driver does not remove his/her foot from the brake pedal after partial braking, deactivation of electronic distribution depending on the brake light switch is rendered impossible. In addition, it may also occur that a defective brake light switch is not identified, or that it is not recognized due to a defective brake light switch that the driver has already terminated pedal-operated braking so that the duration of the electronic brake force distribution is unnecessarily long. This might be the case, for example, when a perceptible deceleration of the vehicle is caused by the engine stall torque without brake pedal application so that the acceleration threshold value is not exceeded.
In order to effect deactivation of electronic brake force distribution and to avoid unnecessary valve actuation also in such cases, an object of the present invention is to provide a brake system of the type referred to hereinabove which causes deactivation of electronic brake force distribution independently of the application of the brake pedal and independently of the exceeding of a given vehicle acceleration as soon as pressure decrease on the rear-axle brakes is not required.